1. Field of the Invention
This invention relates generally to a method and apparatus for optical multiplexing and demultiplexing, and more specifically to a high resolution, wavelength selective optical multiplexer-demultiplexer device.
2. Description of the Prior Art
U.S. patent application Ser. No. 09/193,289, filed Nov. 17, 1998, entitled "COMPACT DOUBLE-PASS WAVELENGTH MULTIPLEXER-DEMULTIPLEXER." describes a multiplexing and demultiplexing device including: a fiber mounting assembly for aligning a plurality of optical fibers; a set of collimating and focusing optics; a transmission grating comprised of a holographic element; and a mirror element. In this device, the fiber mounting assembly assembles a series of substantially close-spaced optical fibers with their ends flushed in a substantially straight line. The set of collimating and focusing optics, transmission grating, and mirror are made and optimized for efficiently operating in preferred communication wavelength regions of the multiplexer/demultiplexer system.
When the described device operates as a demultiplexer, an optical beam containing a plurality of wavelengths is transmitted to the device by an input one of the optical fibers. The divergence of the beam depends on the numerical aperture of the input fiber whose end is located at the vicinity of the focal point of the collimating lens, the lens having a sufficient numerical aperture to accept the diverging beam from the optical fiber. The beam thus is substantially collimated by the lens and then impinged on the holographic element. A plurality of individual wavelengths within the beam are diffracted and angularly separated by the holographic element according to their wavelengths. The spatially separated beams are redirected by the mirror back to the holographic element which provides further spatial separation of the individual wavelengths, thus enhancing the total dispersion of the grating element. The spatially dispersed beams are focused by the aforementioned focusing lens system and received directly by a series of optical fibers.
The described device also operates as a multiplexer, essentially by reversing the beam directions as compared to the beam directions as the device operates in the demultiplexer mode. In the demultiplexer mode, optical beams with different wavelengths from the series of optical fibers are collected by the lens, and then diffracted by the holographic element with specific angular orientations according to the specific wavelength of the individual beam. The diffracted beams are reflected by the mirror, and diffracted again by the holographic element, which eventually merges the beams into a substantially collimated beam including all of the wavelengths. This collimated beam is condensed by the lens and is received by an output one of the optical fibers.
While the device described in U.S. patent application Ser. No. 09/193,289 overcomes the major drawbacks of prior art devices and satisfies several critical requirements, such as providing dense wavelength division multiplexing (DWDM), the device has certain limitations when it is used to handle a large number of channels (e.g., 80 channels). In order to increase the capacity of fiber communication networks, the number of channels can be increased by decreasing channel spacing. For example, for covering a fiber transmitting window of approximately 1540 nm, a 100 GHz (0.8 nm) spacing will generate about 40 channels. Using a 200 GHz (0.4 nm) spacing allows for 80 channels. Moreover, recent advances in fiber-amplification technologies provide for doubling the wavelength range of the fiber transmitting window of approximately 1540 nm, which makes it possible to increase the number of available channels to well over 100 in a nominal spacing. See "Ultrawide 75-nm 3-db Gain-Band Optical Amplification with Erbium-doped Fluoride Fiber Amplifiers and Distributed Raman Amplifiers", IEEE Photonic Technology Letters, Vol. 10, No. 4, April, 1998, by Hiroji Masuda et al.
For the device described in U.S. patent application Ser. No. 09/193,289, because the fiber mounting assembly aligns the optical fibers in a substantially straight line, the end-to-end length of this line of fiber increases as the number of channels increases. Consequently, the optical aperture, and the over-all size of the device must be increased accordingly for two reasons described below.
First, the optical beams closer to the ends of the aforementioned straight line of fibers have an optical axis which diverges from the main optical axis of the device. To avoid the block-out of these beams, the numerical aperture of the collimating and focusing lens must be larger than that of the aforementioned optical fibers by at least a factor of .DELTA./f, where .DELTA. is the distance from the center fiber and end fiber and of the focusing length of the lens. For example, 80 channels of 50/125 multi-mode optical fibers will extend to 5 mm on both sides of the center if arranged in a straight line. For a focusing length with 50 mm focal length, the required numerical aperture of the lens will be at least 0.1 larger than that of the optical fiber. Assuming the focusing length of the lens is fixed, as is normally the case, this increase in the numerical aperture translates into an increase in the diameter of the lens, and therefore an increase in the active area of the transmission grating, the mirror, and the over-all size of the device.
Second, the more the optical beam diverges from the main optical axis of the device, the more likely it suffers optical aberrations. As the number of channel increases, the optical aberrations become significant for the channels closer to the ends. Potential methods for eliminating this off-axis effect include: (1) using a specially designed lens to correct the optical aberrations; and (2) increasing the diameter of the lens (e.g., increase the diameter of the lens to ten times the length of the line of fiber ends). However, these methods are not feasible because a specially designed lens is difficult to manufacture, while increasing the diameter of the lens increases the over-all size of the device. An increase in the size of the aforementioned device will not only lead to increased costs of production, but also make the environmental (thermal, stress etc.) responses of the mechanical and optical assembly of the device hard to control.